Salah Must be conducted while One is free of Unpure Materials in his body, dress.  In addition, Salah needs to be conducted in place free of Unpure substances:

• If one sees a Najasah on him after he finished the Salah, and he is unaware of it, or he is aware of it but he forgot to clean it before the Salah, his Salah is valid and he does not have to repeat it.
• If he forgot and prayed without Wudu, he has to make Wudu and repeat the Salah again.